Percussion drilling is a well-established technique that breaks rock by hammering impacts transferred from the rock drill bit, mounted at one end of a drill string, to the rock at the bottom of the borehole. The energy needed to break the rock is generated by a hydraulically driven piston that contacts a shank adaptor positioned at the opposite end of the drill string to the drill tool. The piston strike on the adaptor creates a stress (or shock) wave that propagates through the drill string and ultimately to the borehole rock bottom.
Shank adaptors typically comprise an internal bore to allow transfer of a flushing fluid to the region of the drill tool. The flushing fluid acts to both cool the tool and to expel drill cuttings and fines from the bore hole. Conventionally, the fluid is introduced into the shank adaptor via a radially extending hole in the adaptor wall that is submerged within a fluid tank that seals onto the external surface of the adaptor axially either side of the hole. Example shank adaptors with internal flushing bores are described in CA 2,247,842; GB 2352671; WO 2012/032485, WO 2008/133584 and WO 2004/079152.
A common problem with existing shank adaptors is the susceptibility for the adaptor wall to fracture with a crack originating and propagating from the flush hole due, in part, to the compressive and tensile stresses generated by the percussive piston and in particular the shock wave that is transmitted through the adaptor to the drill string and ultimately the drill tool. In underground applications, crack initiation is assisted by cavitational damage that exacerbates the problem. Shank adaptor failure is a particular problem for users as it often destroys the rubber seals at the fluid housing surrounding the adaptor. Time consuming replacement in repair of components is required resulting in very undesirable machine downtime. WO 2004/079152 discloses a flushing hole that attempts to reduce the stress at the region of the hole to mitigate fracture. However, there still exists a need for a shank adaptor having a flushing hole that further reduces or eliminates the likelihood of fracture in response to both compressive and tensile forces imparted and transmitted through the adaptor.